Field
Disclosed herein are methods and compositions related to powder metallurgy of titanium and titanium alloys, as well as methods of using these compositions in aircraft, automotive, naval applications, oil equipment, chemical apparatus, and other industries. More particularly, there is disclosed herein methods for the manufacture of near-net shape titanium articles from sintered elemental and alloyed powders. These articles have close size tolerances, which eliminate or minimize the need for machining
Description of Related Art
Titanium alloys are known to exhibit light weight, high resistance to oxidation or corrosion, and the highest specific strength (the strength-to-weight ratio) of all metals except beryllium. Articles of titanium alloys have been produced by melting, forming, and machining processes, or by certain powder metallurgy techniques. However, the first method is not cost effective (although it provides high levels of desired properties of titanium alloys). The second method is cost effective but as previously implemented cannot completely realize all of the desirable advantages of titanium alloys.
Various processes have been developed during the last four decades for the fabrication of near-net shape titanium articles from powders with desirable density and mechanical properties. The use of elemental powder mixtures, control of the particle size distribution, vacuum sintering, hot isostatic pressing, and special surface finishing are among those new developments. But all of these processes, as well as conventional powder metallurgy techniques, impose certain limitations with respect to the characteristics of the produced titanium alloys.
For example, the method described in U.S. Pat. No. 4,432,795 (the contents of which are incorporated herein by reference) includes grinding particles of light metals to a median particle size of less than 20 μm, mixing them with particles of titanium based alloys having a median particle size larger than 40 μm, and compacting the mixture by molding and sintering at temperatures less than that of a formation of any liquid phase. This method allows the manufacture of the alloy having a density close to the theoretical value. However, the resulting alloy, contaminated by oxygen, iron, and other impurities, also exhibits insufficient mechanical properties.
U.S. Pat. No. 4,838,935 (the contents of which are incorporated herein by reference) discloses the use of titanium hydride together with titanium powder in the primary mixture before molding and sintering to form tungsten-titanium sputtering targets. The molded article is heated in a hot-press vacuum chamber to a temperature sufficient for the dehydration of TiH2 to remove gases. Then, the article is heated to a second temperature of 1350-1500° C. while maintaining the pressure and vacuum. This method cannot completely prevent the oxidation of highly-reactive titanium powders during the second heating, because hydrogen is permanently outgassing from the working chamber. Also, the method does not provide sufficient cleaning of titanium powder that resulted in deviations of final products from AMS and ASTM specifications. In addition, this method is not suitable for powdered mixtures containing low-melting metal and phases.
A preliminary partial sintering of titanium and titanium hydride powders with at least one powdered additive of alloying metals (selected from powdered Ni, Al, Cu, Sn, Pd, Co, Fe, Cr, Mn, and Si) is disclosed in U.S. Pat. No. 3,950,166 (the contents of which are incorporated herein by reference). The “mother” alloy obtained in such a way is pulverized and remixed with at least one of powdered titanium or titanium hydride, and optionally with powdered metals such as Mo, V, Zr, and Al—V alloys to achieve the final composition of titanium alloy. This mixture is molded in a predetermined shape and sintered at 1000-1500° C. in a vacuum. While the preliminary sintering partially resolves one technical problem (how to improve uniform distribution of alloying components), the process generates another problem (oxidation of the “mother” powder during pulverization).
Several attempts have been made to improve the density and purity of sintered titanium alloys that involve using titanium hydride as the raw material, together with other alloying powders, e.g., in U.S. Pat. No. 3,472,705, which relates to the production of niobium-titanium or niobium-zirconium superconducting strips. This method includes vacuum heating and sintering accompanied with permanent outgassing, where the heating is used to decompose the hydride to metal before sintering. As a result, the “cleaning effect” of hydrogen is not fully obtained, and partial oxidation reoccurs after the removal of hydrogen from the vacuum chamber. Thus, the method does not provide an effective improvement of mechanical properties of sintered alloys, in spite of any sintering that may be promoted by thermal dissociation of titanium hydride.
A particular process for use of titanium hydride powders combined with master alloy powders or elemental powders has been described in U.S. Patent Application Publication No. 2003/0211001 (the entire contents of which are incorporated herein by reference). However, this publication does not describe a process wherein Commercially Pure (C.P.) titanium powder can be used.
Other known processes for making near-net shape titanium alloys from metal powders have the same drawbacks: (a) insufficient purity and low mechanical properties of sintered titanium alloys, (b) irregular porosity and insufficient density of sintered titanium alloys, and (c) low reproduction of mechanical properties that depend on the purity of raw materials. Indeed, the association of the use of hydrides with increased porosity is so well established that hydrides are specifically disclosed as useful when porous bodies are desirable, as in U.S. Pat. No. 4,560,621.